A substantial amount of property damage is caused every year by trailers hitting objects as they back up. It is very difficult if not impossible for a truck driver to accurately judge the distance between the back of his trailer and the loading dock, and it is often impossible for the driver to see objects that are located directly behind the trailer. As a result, trailers often hit loading docks or other objects, damaging the trailer, the contents in the trailer, or the dock or any other object that has been hit.
There are some devices in the prior art which are designed to hit the dock before the rest of the trailer hits the dock and which sound an alarm when they hit. This is of some use to the driver to help him avoid impact of the trailer with the dock, but, in order to give the driver adequate warning time, it may cause him to stop before the trailer is in the best position. It does not help him bring the trailer smoothly into the proper docking position, which is what the driver really wants and needs.